The present invention relates to a fixing apparatus for an electrostatic copying machine.
In a known electrophotographic process a photoconductive copy sheet is electrostatically charged and radiated with a light image to locally dissipate the charge to form an electrostatic image on the sheet. The sheet is then developed by applying a dry toner substance thereto which adheres to the high charge areas of the electrostatic image to form a toner image. The sheet is then pressingly fed between two fixing rollers which crush the toner into the copy sheet to fix the toner image to the copy sheet.
These fixing rollers are amounted on shafts for rotation, and a biasing force is applied to the ends of the shafts by springs or the like to urge the rollers together. A problem is encountered in such apparatus in that since the force is applied to the ends of the shafts, the shafts and rollers tend to bow or deflect outwardly in such a manner that the pressing force between the rollers is greatest at the ends and least in the center. This causes uneven fixing of the toner to the sheet. In extreme cases the toner will not be sufficiently pressed into the sheet in the center and will come off the sheet onto the hands of persons handling the sheet, objects which the sheets touches, etc.
Two expedients have been introduced in an attempt to overcome this problem. Increasing the diameter of the rollers also increases their rigidity so that the deflection is reduced. However, increasing the diameter of the rollers also increases the size and cost of the copying apparatus to an impractical degree. A second expedient is to orient the axes of the shafts and thereby the rollers so that they form a small actute angle therebetween. Compressive deformation of the rollers due to the pressing force causes a contact portion with the copy sheet in which the pressure distribution is more uniform than with the shafts and rollers aligned parallel to each other. However, a problem exists in this configuration in that the sheets tend to be wrinkled during passage between the fixing rollers unless the angle is so small as to be ineffective. This not only leads to tearing of the sheets in the wrinkled areas but also damages to the rollers due to high stress concentrations caused by the wrinkles.